This command uses a binary search algorithm to find which commit in
your project’s history introduced a bug. You use it by first telling
it a "bad" commit that is known to contain the bug, and a "good"
commit that is known to be before the bug was introduced. Then git
bisect picks a commit between those two endpoints and asks you
whether the selected commit is "good" or "bad". It continues narrowing
down the range until it finds the exact commit that introduced the
change.

In fact, git bisect can be used to find the commit that changed
any property of your project; e.g., the commit that fixed a bug, or
the commit that caused a benchmark’s performance to improve. To
support this more general usage, the terms "old" and "new" can be used
in place of "good" and "bad", or you can choose your own terms. See
section "Alternate terms" below for more information.

Basic bisect commands: start, bad, good

As an example, suppose you are trying to find the commit that broke a
feature that was known to work in version v2.6.13-rc2 of your
project. You start a bisect session as follows:

$ git bisect start
$ git bisect bad # Current version is bad
$ git bisect good v2.6.13-rc2 # v2.6.13-rc2 is known to be good

Once you have specified at least one bad and one good commit, git
bisect selects a commit in the middle of that range of history,
checks it out, and outputs something similar to the following:

Bisecting: 675 revisions left to test after this (roughly 10 steps)

You should now compile the checked-out version and test it. If that
version works correctly, type

$ git bisect good

If that version is broken, type

$ git bisect bad

Then git bisect will respond with something like

Bisecting: 337 revisions left to test after this (roughly 9 steps)

Keep repeating the process: compile the tree, test it, and depending
on whether it is good or bad run git bisect good or git bisect bad
to ask for the next commit that needs testing.

Eventually there will be no more revisions left to inspect, and the
command will print out a description of the first bad commit. The
reference refs/bisect/bad will be left pointing at that commit.

Bisect reset

After a bisect session, to clean up the bisection state and return to
the original HEAD, issue the following command:

$ git bisect reset

By default, this will return your tree to the commit that was checked
out before git bisect start. (A new git bisect start will also do
that, as it cleans up the old bisection state.)

With an optional argument, you can return to a different commit
instead:

$ git bisect reset <commit>

For example, git bisect reset bisect/bad will check out the first
bad revision, while git bisect reset HEAD will leave you on the
current bisection commit and avoid switching commits at all.

Alternate terms

Sometimes you are not looking for the commit that introduced a
breakage, but rather for a commit that caused a change between some
other "old" state and "new" state. For example, you might be looking
for the commit that introduced a particular fix. Or you might be
looking for the first commit in which the source-code filenames were
finally all converted to your company’s naming standard. Or whatever.

In such cases it can be very confusing to use the terms "good" and
"bad" to refer to "the state before the change" and "the state after
the change". So instead, you can use the terms "old" and "new",
respectively, in place of "good" and "bad". (But note that you cannot
mix "good" and "bad" with "old" and "new" in a single session.)

In this more general usage, you provide git bisect with a "new"
commit has some property and an "old" commit that doesn’t have that
property. Each time git bisect checks out a commit, you test if that
commit has the property. If it does, mark the commit as "new";
otherwise, mark it as "old". When the bisection is done, git bisect
will report which commit introduced the property.

To use "old" and "new" instead of "good" and bad, you must run git
bisect start without commits as argument and then run the following
commands to add the commits:

git bisect old [<rev>]

to indicate that a commit was before the sought change, or

git bisect new [<rev>...]

to indicate that it was after.

To get a reminder of the currently used terms, use

git bisect terms

You can get just the old (respectively new) term with git bisect term
--term-old or git bisect term --term-good.

If you would like to use your own terms instead of "bad"/"good" or
"new"/"old", you can choose any names you like (except existing bisect
subcommands like reset, start, …​) by starting the
bisection using

git bisect start --term-old <term-old> --term-new <term-new>

For example, if you are looking for a commit that introduced a
performance regression, you might use

Bisect visualize

To see the currently remaining suspects in gitk, issue the following
command during the bisection process:

$ git bisect visualize

view may also be used as a synonym for visualize.

If the DISPLAY environment variable is not set, git log is used
instead. You can also give command-line options such as -p and
--stat.

$ git bisect view --stat

Bisect log and bisect replay

After having marked revisions as good or bad, issue the following
command to show what has been done so far:

$ git bisect log

If you discover that you made a mistake in specifying the status of a
revision, you can save the output of this command to a file, edit it to
remove the incorrect entries, and then issue the following commands to
return to a corrected state:

$ git bisect reset
$ git bisect replay that-file

Avoiding testing a commit

If, in the middle of a bisect session, you know that the suggested
revision is not a good one to test (e.g. it fails to build and you
know that the failure does not have anything to do with the bug you
are chasing), you can manually select a nearby commit and test that
one instead.

Bisect run

If you have a script that can tell if the current source code is good
or bad, you can bisect by issuing the command:

$ git bisect run my_script arguments

Note that the script (my_script in the above example) should exit
with code 0 if the current source code is good/old, and exit with a
code between 1 and 127 (inclusive), except 125, if the current source
code is bad/new.

Any other exit code will abort the bisect process. It should be noted
that a program that terminates via exit(-1) leaves $? = 255, (see the
exit(3) manual page), as the value is chopped with & 0377.

The special exit code 125 should be used when the current source code
cannot be tested. If the script exits with this code, the current
revision will be skipped (see git bisect skip above). 125 was chosen
as the highest sensible value to use for this purpose, because 126 and 127
are used by POSIX shells to signal specific error status (127 is for
command not found, 126 is for command found but not executable—​these
details do not matter, as they are normal errors in the script, as far as
bisect run is concerned).

You may often find that during a bisect session you want to have
temporary modifications (e.g. s/#define DEBUG 0/#define DEBUG 1/ in a
header file, or "revision that does not have this commit needs this
patch applied to work around another problem this bisection is not
interested in") applied to the revision being tested.

To cope with such a situation, after the inner git bisect finds the
next revision to test, the script can apply the patch
before compiling, run the real test, and afterwards decide if the
revision (possibly with the needed patch) passed the test and then
rewind the tree to the pristine state. Finally the script should exit
with the status of the real test to let the git bisect run command loop
determine the eventual outcome of the bisect session.

OPTIONS

--no-checkout

Do not checkout the new working tree at each iteration of the bisection
process. Instead just update a special reference named BISECT_HEAD to make
it point to the commit that should be tested.

This option may be useful when the test you would perform in each step
does not require a checked out tree.

Here we use a test.sh custom script. In this script, if make
fails, we skip the current commit.
check_test_case.sh should exit 0 if the test case passes,
and exit 1 otherwise.

It is safer if both test.sh and check_test_case.sh are
outside the repository to prevent interactions between the bisect,
make and test processes and the scripts.

Automatically bisect with temporary modifications (hot-fix):

$ cat ~/test.sh
#!/bin/sh
# tweak the working tree by merging the hot-fix branch
# and then attempt a build
if git merge --no-commit hot-fix &&
make
then
# run project specific test and report its status
~/check_test_case.sh
status=$?
else
# tell the caller this is untestable
status=125
fi
# undo the tweak to allow clean flipping to the next commit
git reset --hard
# return control
exit $status

This applies modifications from a hot-fix branch before each test run,
e.g. in case your build or test environment changed so that older
revisions may need a fix which newer ones have already. (Make sure the
hot-fix branch is based off a commit which is contained in all revisions
which you are bisecting, so that the merge does not pull in too much, or
use git cherry-pick instead of git merge.)